Ejector Device for Forming a Pressurized Mixture of Liquid and Gas, and Use Therefore

ABSTRACT

Ejector device for forming a pressurized mixture of liquid and gas, comprising a suction chamber and a diffuser. The suction chamber comprises an injection nozzle for producing a jet of liquid flowing in a longitudinal direction, a gas inlet for admitting into the suction chamber a gas to be driven by the liquid jet, and an outlet opening for discharging the liquid jet and the driven gas from the suction chamber. The diffuser is connected to the outlet opening of the suction chamber and has, in the longitudinal direction, a transversal section that increases from the outlet opening, the diffuser being situated immediately after the outlet opening of the suction chamber.

The present invention relates to an ejector device for forming a pressurized mixture of liquid and gas.

The document WO-01/34285 describes such an ejector device comprising a suction chamber, a cylindrical tube and a conical-shaped diffuser that widens in a longitudinal direction. A nozzle injects a liquid at high speed into the suction chamber, which then sucks gas through an inlet. The cylindrical tube is situated between the suction chamber and the diffuser, so that the liquid and the gas are mixed in this cylindrical tube before entering into the diffuser.

Such an ejector device makes it possible to obtain compression rates (see definition below) of the order of 4 to 8. Thus, a gas that has a pressure of 2 atm at the inlet can be compressed to a pressure of 16 atm. It is very difficult to go beyond that.

The aim of the present invention is to refine an ejector device of this type, notably to optimize its energy efficiency and increase the compression rate.

More particularly, the invention relates to an ejector device for forming a pressurized mixture of liquid and gas, comprising a suction chamber and a diffuser, wherein the suction chamber comprises:

-   -   an injection nozzle for producing a jet of liquid flowing in a         longitudinal direction;     -   a gas inlet for admitting into the suction chamber a gas to be         driven by the liquid jet; and     -   an outlet opening for discharging the liquid jet and the driven         gas from the suction chamber;         wherein the diffuser is connected to the outlet opening of the         suction chamber and has, in the longitudinal direction, a         transversal section that increases from said outlet opening, the         diffuser with increasing section being situated immediately         after the outlet opening of the suction chamber, and wherein the         diffuser (6) comprises at least one first conical portion that         has a first angle of between 0.1 and 7 degrees.

Thanks to these arrangements, the liquid and gas mixture can be produced at different axial positions inside the diffuser, and the ejector device then makes it possible to operate over a wide range of compression rates.

In various embodiments of the ejector device according to the invention, it is possible, if necessary, to also use one and/or other of the following arrangements:

-   -   the first angle is preferentially between 1.5 and 4 degrees;     -   the diffuser also comprises a second conical portion continually         in extension of the first portion in the longitudinal direction,         said second portion having a second angle greater than the first         angle;     -   the second angle is between 5 and 15 degrees, and preferably of         the order of 7 degrees;     -   the diffuser also comprises a second portion continually in the         extension of the first portion in the longitudinal direction,         said second portion having a convex profile shape;     -   the convex second portion has an angle that progressively         increases in the longitudinal direction from the first angle to         an angle less than 15 degrees, and preferably of the order of 10         degrees;     -   the diffuser is substantially coaxial to the injection nozzle         and to the outlet opening of the suction chamber;     -   the ejector device is such that:         -   the outlet opening, also called neck, has a neck surface             S_(c) perpendicular to the longitudinal direction,         -   the injection nozzle has a nozzle surface S₂ inside the             nozzle and perpendicular to the longitudinal direction, and         -   a geometrical ratio R is the ratio between the nozzle             surface S₂ and the neck surface S_(c), said geometrical             ratio R being between 0.5 and 0.9;

Thanks to this arrangement, the device makes it possible to maximize the compression rate for a given injection speed, and in particular to reach mixture compression rates that are very high, and for example greater than 30, with a single device stage, provided that the speed of the liquid jet is sufficiently high;

-   -   the ejector device is such that:         -   the injection nozzle has one end in the longitudinal             direction,         -   the outlet opening has a circular section with a neck             diameter Dc, and         -   the end is situated at a retraction distance x₂ from the             outlet opening, said retraction distance x₂ being between             one and five times the neck diameter Dc;     -   the suction chamber comprises walls in the longitudinal         direction extending radially in said suction chamber, so that         the gas flows in the suction chamber with a flow with little         turbulence, without rotation, with fairly uniform axial speed         distribution;     -   the injection nozzle comprises liquid channelling means suitable         for obtaining, in the nozzle after said channelling means, a         flow of the liquid with little turbulence, without rotation and         with substantially uniform axial speed distribution;     -   the liquid channelling means in the nozzle are chosen from:         -   a device that has walls extending in the longitudinal             direction, and         -   a device that has walls extending in the longitudinal             direction, said walls having a honeycomb shape, and         -   a device comprising a wall in a direction that is             substantially perpendicular to the longitudinal direction             and comprising holes for distributing the liquid flow in a             substantially uniform manner in the transversal section of             the nozzle.

The invention also relates to the use of an ejector device of the preceding type, wherein:

-   -   the following are measured: the gas suction pressure p₁ at the         gas inlet (3), the liquid feed pressure p₂ feeding the injection         nozzle (5), the discharge pressure p₃ of the gas and liquid         mixture downstream of the diffuser (6), and     -   at least one of said pressures is set so that a compression         parameter Ψ by the following formula:

${\Psi = \frac{p_{3} - p_{1}}{p_{2} - p_{1}}},$

is between 0.4 and 0.6.

The invention also relates to the use of an ejector device of the preceding type, wherein:

-   -   the following are measured: the gas suction pressure p₁ at the         gas inlet (3), the liquid feed pressure p₂ feeding the injection         nozzle (5), the discharge pressure p₃ of the gas and liquid         mixture downstream of the diffuser (6), and     -   the liquid feed pressure p₂ is set to plus or minus twenty         percent of an optimal pressure p_(2,opt), such that:

p _(2,opt)=2.p ₃ −p ₁.

Thanks to these usage arrangements, the energy performance levels of the ejector device are optimized.

The invention can, for example, be used in a gas compressor comprising an ejector device fed with a gas on the one hand and a liquid on the other hand, and a separator device suitable for receiving a liquid and gas mixture originating from the ejector device and extracting a gaseous component from this mixture, wherein the ejector device comprises a suction chamber and a diffuser, wherein the suction chamber comprises:

-   -   an injection nozzle for producing a jet of liquid flowing in a         longitudinal direction;     -   a gas inlet for admitting a driven gas into the suction chamber;         and     -   an outlet opening for discharging the liquid jet and the driven         gas from the suction chamber;         wherein the diffuser is connected to the outlet opening of the         suction chamber and has, in the longitudinal direction, a         transversal section that increases from said outlet opening, the         diffuser with increasing section being situated immediately         after the outlet opening of the suction chamber, and wherein the         gas separator device comprises two outlets, one for the gas and         the other for the liquid.

In various embodiments of the gas compressor, it is possible, if necessary, to also use one or other of the following arrangements:

-   -   the diffuser comprises at least a first conical portion that has         a first angle between 0.1 and 7 degrees;     -   the separator device is a gravity separator;     -   the separator device is a cyclonic separator;     -   the gas compressor also comprises a pump suitable for sucking         the liquid under pressure at the level of the separator device         and for feeding the injection nozzle of the ejector device with         said liquid.

Other features and benefits of the invention will become apparent from the following description of one of its embodiments, given by way of nonlimiting example, in light of the appended drawings.

In the drawings:

FIG. 1 is a diagrammatic view in longitudinal cross section of the ejector device according to the invention,

FIG. 2 is a graph, established from experimental results, showing the driving rate τ_(e) (see definition below) as a function of the compression rate τ_(c) (see definition below) for different values of the gas suction pressure p₁, in the ejector device of FIG. 1,

FIG. 3 is a graph showing the theoretical efficiency of the ejector device (see definition below) of FIG. 1, for a compression rate of the order of 4, as a function of a geometrical ratio R for different driving rate values,

FIG. 4 is a graph showing the efficiency of the ejector device of FIG. 1, as a function of a compression parameter Ψ for different values of the motive pressure parameter χ (see definition below)

FIG. 5 is a diagrammatic view of a gas compressor comprising the ejector device of FIG. 1.

The longitudinal direction mentioned in this description should be understood to be the direction indicated by a chain dotted line X in FIG. 1, and corresponds to the direction of flow in the ejector device 1 between the upstream side situated on the left and the downstream side situated on the right in this figure.

FIG. 1 is a diagrammatic view in longitudinal cross section of an ejector device 1 according to the invention. This ejector device extends along the longitudinal axis X and comprises, along this axis:

-   -   a suction chamber 2 suitable for sucking a gas by the injection         of a liquid jet at high speed into said suction chamber 2, and     -   a diffuser 6 suitable for mixing the liquid and the gas and         compressing this mixture, fairly abruptly, by a phenomenon         similar to a hydraulic jump, then progressively compressing this         mixture by converting the kinetic energy of the mixture into         pressure energy.

The suction chamber 2 comprises:

-   -   a lateral inlet opening 3 through which the gas is brought,     -   an injection nozzle 5 ending in a cylindrical tube substantially         coaxial to the longitudinal axis X and opening into said suction         chamber, and through which a liquid is injected at high speed         into said suction chamber, and     -   an outlet opening 4 opposite the nozzle 5 in the direction of         flow, coaxial to the longitudinal axis X.

The outlet opening 4 therefore forms, at the outlet of the suction chamber 2, a constriction which is also called neck. The outlet opening 4 has a substantially circular section of diameter D_(c). It has a neck surface S_(c), S_(c)=π·D_(c) ²/4, perpendicular to the longitudinal axis X.

A first upstream duct 3 a feeds the inlet opening 3 of the suction chamber 2 with gas, at a suction pressure p₁ with a volume flow rate Q₁.

A second upstream duct 5 a feeds the injection nozzle 5 with liquid, at a feed pressure p₂ with a volume flow rate Q₂.

The nozzle 5 has an end 5 b in the suction chamber 2, of internal diameter D₂ and having a nozzle surface S₂, S₂=π·D₂ ²/4, perpendicular to the longitudinal axis X. This end 5 b is placed at a retraction distance x₂ from the outlet opening 4 of the suction chamber 2. The internal diameter D₂ of the end 5 b is possibly less than an internal diameter of the nozzle 5, so that said nozzle has, at its end 5 b, a contracted section.

The injection nozzle 5 possibly includes liquid channelling means that is suitable for obtaining, in the nozzle after said channelling means, a flow of the liquid with little turbulence, without rotation and with substantially uniform axial speed distribution, that is to say, with an axial speed distribution in a transversal section of the nozzle that is substantially constant. The liquid jet produced by the nozzle 5 in the suction chamber then remains substantially cylindrical as far as the outlet opening 4 of said chamber. Thus, the liquid jet diverges little in this chamber and does not begin to be mixed with the gas before the diffuser 6. Usually, those skilled in the art consider that having a divergent liquid jet helps in forming a liquid and gas mixture. As it happens, the inventors have discovered that, on the contrary, this arrangement makes it possible to obtain a better liquid and gas mixture in the diffuser 6 and a better compression rate of this mixture.

The channelling means of the liquid in the nozzle 5 can, for example, be a device that has walls extending in the longitudinal direction X, or a device that has walls extending in the longitudinal direction X and said walls having a honeycomb shape, or a device comprising a wall in a direction substantially perpendicular to the longitudinal direction X and having holes for distributing the liquid flow in a substantially uniform manner in the transversal section of the nozzle, or a combination of these devices in the nozzle 5 and arranged one after the other in the longitudinal direction X.

The channelling means can then be placed in the nozzle at a short distance from its end 5 b, for example at a distance of between 10 and 30 times the internal diameter D₂ of the nozzle 5, and preferably equal to 20 times this diameter.

The diffuser 6 is mounted in the extension of the outlet opening 4 of the suction chamber. This diffuser 6 has, in the longitudinal direction X, a transversal section that increases from said outlet opening 4. This diffuser 6 is, for example, conical in shape, widening in the direction of flow, and is also substantially coaxial to the longitudinal axis X. It therefore has an upstream diameter substantially equal to the diameter D_(c) of the outlet opening 4 of the suction chamber 2, and a downstream diameter D₃ greater than the upstream diameter D_(c). The diffuser 6 forms a cone that has an angle α_(d). The angle α_(d) is defined as the total diffuser angle of the cone, and has a low value, at least in a first portion of the diffuser 6.

A downstream duct 6 a supplies, at the outlet, the liquid and gas mixture at the discharge pressure p₃.

Unlike the prior art devices, the inventive ejector device 1 has a diffuser 6 situated immediately at the outlet of the suction chamber 2, that is to say without the interposition of a cylindrical tube for mixing the liquid and the gas, so that the mixture is produced directly in the diffuser 6.

The inventors have confirmed that such an arrangement would enable the ejector device 1 to operate over a wide range of compression rates τ_(c).

The compression rate τ_(c) is defined as being the ratio between the discharge pressure p₃ and the suction pressure p₁ of the gas:

$\tau_{c} = \frac{p_{3}}{p_{1}}$

The driving rate τ_(c) is defined as being the ratio between the volume flow rate Q₁ of the driven gas at the inlet opening 3 and the volume flow rate Q₂ of the liquid injected through the injection nozzle 5:

$\tau_{e} = \frac{Q_{1}}{Q_{2}}$

The motive pressure parameter χ is defined as being the ratio between the liquid feed pressure p₂ feeding the injection nozzle 5 and the gas suction pressure p₁:

$\chi = \frac{p_{2}}{p_{1}}$

These adimensional parameters, which can be determined by calculation or measurement on test devices, make it possible to establish dimensioning laws to optimize the operation of the device.

Tests have shown that the driving rate τ_(e) is linked to the compression rate τ_(c). The curves of FIG. 2 show this dependency for several gas suction pressure values p₁.

The ejector device 1 operates as follows.

The liquid goes into the suction chamber 2 at the end 5 b of the nozzle 5, at a pressure equal to the gas suction pressure p₁ and at a speed U₂. It forms a rectilinear and substantially cylindrical jet inside the suction chamber 2. This high speed jet helps to drive the gas which surrounds the jet towards the outlet opening 4 of said suction chamber 2. We therefore have, in the suction chamber, two substantially separate phases: a liquid phase, the section of which is a disc, close to the longitudinal axis X, and a gaseous phase, the section of which is a ring in contact with said disc, at a certain distance from this longitudinal axis and coaxial to the liquid phase.

The suction chamber 2 possibly comprises, from said distance from the longitudinal axis X, walls that extend radially and longitudinally, so that the liquid jet does not come into contact with said walls and the gas contained in this suction chamber 2 is driven with a flow with little turbulence, without rotation and with substantially uniform axial speed distribution towards the outlet opening 4 of the suction chamber 2.

In the diffuser 6, the flow comprises, along the axis X, a first, a second and then a third area. In the first area of the flow, the two coaxial phases flow in a relatively separate manner. In the second area of flow, called mixing area, the flow changes structure fairly abruptly and becomes an increasingly uniform mixture of the liquid and of the gas. This change of structure of the flow is accompanied by a fairly abrupt slowing down of the liquid phase and an increase in pressure. In the third area of flow, the two phases flow in the form of a finely mixed emulsion. In this third area, the flow slows down progressively under the effect of the increasing section of the diffuser. The kinetic energy of the mixture is then converted into pressure energy.

These first, second and third areas of the flow are not separated by clear and distinct transitions, the phenomena being continuous. Also, these areas of the flow can be displaced longitudinally in the diffuser 6, notably by the effect of variations of the discharge pressure p₃ downstream of the diffuser 6. Despite such variations, the operation of the ejector device is little disturbed, which shows that such a device is stable and tolerant to the variations of the operating parameters.

In a simplified manner, the quantity of movement of the liquid jet at the inlet of the diffuser 6 is converted into pressure forces that are applied either side of the mixing area. If we draw an analogy with the compressible flows, this conversion can be seen as a shock. If we draw an analogy with free surface flows, this conversion can be seen as a hydraulic jump.

The conical-shaped diffuser 6 has an angle α_(d) that is small, but not zero. A conical diffuser 6 with a greater angle α_(d), for example greater than 10 degrees, does not provoke such an effective hydraulic shock and does not make it possible to achieve such high compression rates.

The inventors have therefore confirmed that there is an optimum angle α_(d,opt) for which the compression rate is maximum, for a given injection speed U₂. This optimum angle lies within a range of angle values α_(d) between 0.1 and 7 degrees, and preferably between 1.5 and 4 degrees. The value of the optimum angle α_(d,opt) is difficult to determine by prior calculation.

In a variant of the ejector device 1, the diffuser 6 comprises, along the axis X, a first conical portion with a first angle α_(d), then a second conical portion with a second angle. The second portion is continually in the extension of the first portion. The second angle is greater than the first angle. The second angle can be between 5 and 15 degrees, and preferably of the order of 7 degrees. The first portion is intended to accommodate the mixing area, which should take place under a low divergence angle in order to maximize the compression rate. The second portion ensures the final recovery of pressure by conversion of the kinetic energy of the mixture. This energy conversion can take place under a greater divergence angle, for example of the order of 10°, without in any way causing a significant pressure drop. There are therefore obtained both a high compression rate τ_(c) through the first portion with low divergence angle, and a shortened overall length of the diffuser 6.

In another variant of the ejector device 1, the diffuser 6 has a flared shape with a first portion of conical shape with a small first angle, then, in continuity, a shape with a convex profile. The second convex portion has an angle that increases progressively in the longitudinal direction X from the first angle to an angle, for example less than 15 degrees, and preferably of the order of 10 degrees. The overall length of the diffuser 6 can thus be further shortened without affecting the compression rate.

In yet another variant of the ejector device 1, the diffuser 6 has a flared shape with a shape that has a convex profile, said convex profile having an angle that increases progressively in the longitudinal direction X from a first angle α_(d) to an angle, for example less than 15 degrees, and preferably of the order of 10 degrees. The overall length of the diffuser 6 can thus be shortened further.

The first angle α_(d) of the preceding variants advantageously has a value within the range from 0.1° to 7°, as indicated hereinabove.

Furthermore, the efficiency η of the ejector device is the ratio between the compression power P_(c) in the ejector device 1 and the hydraulic power P_(h) supplied.

If we assume that the compression is substantially isothermic, we obtain the following compression power P_(c):

$P_{c} = {p_{1}Q_{1}{\ln \left( \frac{p_{3}}{p_{1}} \right)}}$

When a pump sucks the liquid at the level of the separator situated at the discharge of the ejector device 1, the supplied hydraulic power P_(h) is linked to the difference of liquid feed pressure p₂ in the injection nozzle 5 and the discharge pressure p₃ at the outlet of the diffuser 6, that is to say:

P _(h) =Q ₂(p ₂ −p ₃)

hence the following efficiency η:

$\eta = {\frac{Q_{1}}{Q_{2}}\frac{p_{1}}{p_{2} - p_{3}}{\ln \left( \frac{p_{3}}{p_{1}} \right)}}$

that can be expressed as a function of the adimensional parameters defined previously:

$\eta = {\tau_{e}\frac{\ln \left( \tau_{c} \right)}{\chi - \tau_{c}}}$

The efficiency η of an ejector device 1 can therefore be measured on experimental devices, or be calculated by a mathematical hydraulic flow model.

The adimensional geometrical ratio R has also been defined as being the ratio of the nozzle surface S₂ relative to the neck surface S_(c):

$R = \frac{S_{2}}{S_{c}}$

As shown by the theoretical curves of FIG. 3, given a fixed driving rate, the efficiency η is linked to this geometrical ratio R of the ejector device 1. The efficiency η is maximum for a geometrical ratio R between 0.5 and 0.9, or more specifically between 0.6 and 0.8. This trend has been confirmed by experimental results.

Experimental tests have also shown that the optimum retraction distance x₂ for the targeted compression rates is from one to five times the neck diameter D_(c) of the outlet opening 4 of the ejector device 1.

Another dimensioning criterion has been defined by introducing a new adimensional parameter Ψ, called compression parameter, and defined as follows:

$\Psi = \frac{p_{3} - p_{1}}{p_{2} - p_{1}}$

A first benefit of this compression parameter Ψ is that it can be calculated only with the pressure values, which can be measured on an experimental ejector device.

This compression parameter Ψ can be expressed as a function of the other adimensionnal parameters by the following expression:

$\Psi = \frac{\tau_{c} - 1}{\chi - 1}$

For a given injection speed U₂, the efficiency η is linked to the value of this compression parameter Ψ of the ejector device 1. The curves of FIG. 4 show this dependency for several values of the motive pressure parameter χ. The efficiency η is then maximum for a compression parameter Ψ that lies within the range from 0.4 to 0.6, or preferably equal to approximately 0.5.

A second benefit of this compression parameter Ψ is that, conversely, it can make it possible to determine the liquid feed pressure p₂ that is suitable for obtaining the optimum efficiency η_(opt) of the ejector device 1.

In practice, the above range for the compression parameter Ψ makes it possible to determine that the liquid feed pressure p₂ should be within the following range:

1.66·p ₃−0.66·p ₁ <p ₂<2.5·p ₃−1.5·p ₁

with an optimum central liquid feed pressure value p_(2,opt) of:

p _(2,opt)=2·p ₃ −p ₁

The ejector device 1 can then be used in a gas compressor 10 as shown in FIG. 5.

This gas compressor 10 comprises:

-   -   a gas inlet 11 at low pressure,     -   a gas outlet 12 at high pressure,     -   a looped internal hydraulic circuit.

The hydraulic circuit comprises, in series:

-   -   an ejector device 1 fed on the one hand with a low pressure gas,         originating from the gas inlet 11, and on the other hand with a         high pressure liquid; said ejector device 1 supplying a mixture         of gas and liquid at intermediate pressure,     -   a separator device 13 fed with a mixture of gas and liquid by         the ejector device 1 and supplying on the one hand a gas         component to the gas outlet 12 at intermediate pressure and a         liquid, at the same intermediate pressure, to a return circuit         14,     -   a heat exchanger 15 in the return circuit 14 suitable for         maintaining the temperature of the hydraulic circuit at an         appropriate level,     -   a pump 16 fed by the liquid from the return circuit 14 and         supplying a liquid at higher pressure to a feed circuit 17.

The feed circuit 17 then feeds the ejector device 1 of the gas compressor 10 with liquid.

The separator device 13 is either a gravity separator or a cyclonic separator.

Furthermore, a branched circuit 14 a circumvents the heat exchanger 15 of the return circuit 14 and includes a valve 14 b. This branch circuit 14 a is suitable for adjusting the temperature of the hydraulic circuit.

The heat exchanger 15 is also fed with a cold fluid, for example water, by a cooling circuit 15 a and a pump 15 b.

The gas compressor 10 operates as follows.

The ejector device 1 mixes the gas with a liquid injected at high speed, and compresses this mixture of gas and liquid at a high pressure. The mixture is separated in the separator device 13, which then supplies the gas outlet 12 with a gas at high pressure, and the return circuit 14 with a liquid that is also at high pressure. The heat exchanger 15 makes it possible to extract heat from the liquid. The pump 16 increases the pressure of the liquid before feeding the feed circuit 17 and the ejector device 1. As already explained above, the ejector device 1 comprises an injection nozzle suitable for injecting said liquid into its suction chamber at high speed.

Thus, the injection nozzle of the ejector device 1 produces an expansion of the liquid (transformation of the pressure energy of the liquid into kinetic energy). The diffuser of the injection device 1 mixes and compresses the mixture. The pump 16 complements the compression of the liquid to achieve the feed pressure at the inlet of the nozzle of the ejector device. 

1. An ejector device for forming a pressurized mixture of liquid and gas, comprising a suction chamber and a diffuser, wherein the suction chamber comprises: an injection nozzle for producing a jet of liquid flowing in a longitudinal direction (X); a gas inlet for admitting into the suction chamber a gas to be driven by the liquid jet; and an outlet opening for discharging the liquid jet and the driven gas from the suction chamber; wherein the diffuser is connected to the outlet opening of the suction chamber and has, in the longitudinal direction, a transversal section that increases from said outlet opening, the diffuser with increasing section being situated immediately after the outlet opening of the suction chamber, and wherein the diffuser comprises at least one first conical portion that has a first angle of between 0.1 and 7 degrees.
 2. An ejector device according to claim 1, wherein the first angle is between 1.5 and 4 degrees.
 3. An ejector device according to claim 1, wherein the diffuser also comprises a second conical portion continually in the extension of the first portion in the longitudinal direction, said first portion having a second angle greater than the first angle.
 4. An ejector device according to claim 3, wherein the second angle is between 5 and 15 degrees, and preferably of the order of 7 degrees.
 5. An ejector device according to claim 1, wherein the diffuser also comprises a second portion continually in the extension of the first portion in the longitudinal direction, said second portion having a convex profile shape.
 6. An ejector device according to claim 5, wherein the convex second portion has an angle that progressively increases in the longitudinal direction from the first angle to an angle less than 15 degrees, and preferably of the order of 10 degrees.
 7. An ejector device according to claim 1, wherein the diffuser is substantially coaxial to the injection nozzle and to the outlet opening of the suction chamber.
 8. An ejector device according to claim 1, wherein: the outlet opening has a neck surface S_(c) perpendicular to the longitudinal direction, the injection nozzle has a nozzle surface S₂ inside the nozzle and perpendicular to the longitudinal direction, and a geometrical ratio R is the ratio between the nozzle surface S₂ and the neck surface S_(c), said geometrical ratio R being between 0.5 and 0.9.
 9. An ejector device according to claim 1, wherein: the injection nozzle has one end in the longitudinal direction, the outlet opening has a circular section with a neck diameter D_(c), and the end is situated a retraction distance x₂ from the outlet opening, said retraction distance x₂ being between one and five times the neck diameter D_(c).
 10. An ejector device according to claim 1, wherein the suction chamber comprises walls in the longitudinal direction extending radially in said suction chamber, so that the gas flows in the suction chamber with a flow with little turbulence, without rotation and with substantially uniform axial speed distribution.
 11. An ejector device according to claim 1, wherein the injection nozzle comprises liquid channelling means suitable for obtaining, in the nozzle after such channelling means, a flow of the liquid with little turbulence, without rotation and with substantially uniform axial speed distribution.
 12. An ejector device according to claim 11, wherein the liquid channelling means in the nozzle are chosen from: a device that has walls extending in the longitudinal direction, and a device that has walls extending in the longitudinal direction, said walls having a honeycomb shape, and a device comprising a wall in a direction that is substantially perpendicular to the longitudinal direction and comprising holes for distributing the liquid flow in a substantially uniform manner in the transversal section of the nozzle.
 13. Use of an ejector device according to claim 1, wherein: the following are measured: a gas suction pressure p₁ at the gas inlet, a liquid feed pressure p₂ feeding the injection nozzle, a discharge pressure p₃ of the gas and liquid mixture downstream of the diffuser, and at least one of said pressures is set so that a compression parameter defined by the following formula: ${\Psi = \frac{p_{3} - p_{1}}{p_{2} - p_{1}}},$ is between 0.4 and 0.6.
 14. Use of an ejector device according to claim 1, wherein: the following are measured: a gas suction pressure p₁ at the gas inlet, a liquid feed pressure p₂ feeding the injection nozzle, a discharge pressure p₃ of the gas and liquid mixture downstream of the diffuser, and said liquid feed pressure p₂ is set to plus or minus twenty percent of an optimal pressure p_(2,opt), such that: p _(2,opt)=2·p ₃ −p ₁. 